The accurate measurement of downhole fluid pressure and temperature in a borehole has long been recognized as being important in the production of oil, gas, and/or geothermal energy. Accurate pressure and temperature measurements are important in maximizing the efficiency of a well and may indicate problems in oil recovery operations. Both secondary hydrocarbon recovery operations and geothermal operations typically require pressure and temperature information to determine various factors considered useful in predicting the success of the operation, and in obtaining the maximum recovery of energy from the borehole.
In secondary hydrocarbon recovery operations, accurate borehole pressure specifically give an indication of well productivity potential, and allow the operator to predict the amount of fluid that should be required to fill the formation before oil or gas can be expected to be forced out from the formation into the borehole and then recovered to the surface. The accurate measurement of pressure and temperature changes in well fluids from each of various boreholes extending into a formation may indicate the location of injection fluid fronts, as well as the efficiency with which the fluid front is sweeping the formation. In geothermal wells, accurate pressure and temperature information is critical to efficient production due to the potential damage which occurs if reinjected fluids cool the formation or changes in fluid dynamics cause well bore plugging.
Techniques have been devised for providing a periodic measurement of downhole conditions by lowering sensors into the borehole at desired times, although such periodic measurement techniques are both inconvenient and expensive due to the time and expense normally required to insert instrumentation into the borehole. Any such periodic measurement technique is limited in that it provides only a representation of borehole conditions at specific times, and does not provide the desired information over a substantial length of time which is typically desired by the operator.
Permanent installation techniques have been devised for continually monitoring pressure in a borehole in a manner which overcomes the inherent problems associated with periodic measurement. One such prior art technique employs a downhole pressure transducer and a temperature sensor having electronic scanning ability for converting detected downhole pressures and temperatures into electronic data, which then are transmitted to the surface on a conductor line. The conductor line is normally attached to the outside of the tubing in the wellbore, and the transducer and temperature sensor are conveniently mounted on the lower end of the production tubing. This system has shortcomings, however, in part because of the expense and high maintenance required for the electronics positioned in the hostile wellbore environment over an extended period of time. The high temperatures, pressures and/or corrosive fluids in the wellbore substantially increase the expense and decrease the reliability of the downhole electronics. Downhole pressure transducers and temperature sensors which output electronic data for transmission to the surface are generally considered delicate systems, and thus are not favored in the hostile environments which normally accompany a downhole wellbore.
Overcoming these problems, a system for downhole pressure measurement was devised utilizing a small diameter capillary tube or microtube connected to a downhole pressure chamber. The pressure chamber is in fluid communication with the fluid pressure in the well. The small diameter tubing transmits the pressure from the downhole location to the surface where pressure measurement using conventional or electronic pressure gauges is possible in a friendlier environment. These systems are sometimes referred to as Pressure Telemetry Systems or Molecular Telemetry Systems. Typically a monitoring gas, such as helium or nitrogen, used. U.S. Pat. No. 3,895,527, issued to McArthur, incorporated herein by reference for all purposes, discloses a system for remotely measuring pressure in a borehole which utilizes a small diameter tube which has one end exposed to borehole pressure and has its other end connected to a pressure gauge or other detector at the surface.
The concept of measuring downhole pressure according to a system which uses such a small diameter tube is also disclosed in U.S. Pat. No. 3,898,877, issued to McArthur, and an improved version of such a system is disclosed in U.S. Pat. No. 4,010,642, also issued to McArthur, both of which are incorporated herein by reference for all purposes. The teachings of this latter patent have rendered this technology particularly well suited for more reliably measuring pressure in a borehole, since the lower end of the tube extends into a chamber having at least a desired volume. Further methods are found in U.S. Pat. No. 4,505,155 to Jackson, incorporated herein by reference for all purposes. U.S. Pat. No. 4,018,088 to McArthur teaches use of a downhole high pressure float valve in the chamber. Accurate downhole temperature readings in conjunction with pressure readings utilizing small diameter tubing pressure transmission are taught in U.S. Pat. Nos. 4,976,142 and 5,163,321, both issued to Perales and both incorporated herein for all purposes. Additional improvements have been made resulting in retrievable pressure telemetry systems, purging and system check techniques, simultaneous temperature measurement, advanced temperature and pressure measurement techniques, expandable chambers, continuous capillary tubing, capillary gas weight calculation to correct for truer bottom hole pressures, use of helium as the monitoring gas, concentric chambers, automatic purge systems and others. Pressure telemetry systems are commercially available from Halliburton Energy Services under the tradename EZ-Gauge.
One problem with the pressure telemetry systems is the lack of a device to stop hydrocarbon flow up the small diameter conduit in the case of failure of the system due to a leak of the monitoring gas or due to a catastrophic wellhead event. The continuous conduit of molecules to the surface is perfectly safe during normal operation, but can become a concern after catastrophic events. If the wellhead is severely damaged, such as after it is hit by a truck or other surface equipment, by a natural or man-made disaster, such as an iceberg, tsunami, hurricane, tornado, avalanche, earthquake, mudslide or military ordnance, the conduit can become a potential path for hydrocarbon to travel from the wellbore to the surface. Due to the extremely small diameter of the conduit, the surface leak will be small or even non-existent if the conduit becomes plugged, but the potential does exist for a leak. Whether the failure of the system is due to a catastrophic event or a leak in the conduit, wellbore fluid flows into the conduit where it can foul the small diameter tubing of the conduit.
Disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for reducing or eliminating the possibility of a surface leak after a catastrophic wellhead event and preventing movement of wellbore fluid into the small diameter tubing of a pressure telemetry system.